During the manufacture of pigmented coatings or inks, the most labor intensive and time consuming step is the formation of the pigment dispersion. This is especially true with very transparent coatings using organic pigments for waterborne systems, which require extended milling times to reduce pigment particle sizes. In addition, the resulting pigment must be easily dispersible in systems, particularly in stir-in systems.
Many dispersants have been developed to help minimize milling time and stabilize pigment dispersions. Such dispersants range from low molecular weight surfactants to high molecular weight polymers. Among the more efficient polymeric dispersants are those based on polyurethane. The chemistry of the polyurethane dispersants has been extensively modified to include non-ionic, anionic, and cationic types. Many of these work well to stabilize liquid pigment dispersions, but not much has been explored to use polyurethane dispersants for the surface treatments of dry pigments. GB 1111223 discloses a colored polyurethane elastomeric particle, which is formed in the presence of a pigment and is applicable for non-transparent paints. U.S. Pat. No. 4,844,742 discloses polyurethane dispersants (Byk 160), which are designed for solvent-borne paints and used as pigment surface treatments; however, additional milling is still required to prepare a dispersion. Similarly, U.S. Pat. No. 4,631,330 uses a solvent solution of polyurethane to surface-treat a pigment, which is then dried. Although this preparation disperses the pigment well in an alkyd-melamine solvent-borne coating, some additional milling is still required to prepare the paints. U.S. Patent Application Publication Nos. 2004/097685 and 2005/004284 disclose polyurethane polymers which work well in water-borne coatings, but some additional milling is still required to prepare the paints. U.S. Pat. No. 6,063,182 discloses stir-in compositions with relatively opaque pigments, using polymers or co-polymers of vinylpyrrolidone, which is subsequently spray dried. These dispersants sometimes have compatibility problems and negatively affect coating properties in some systems. Pigments require high energy in the milling process.
Thus, there exists a need for easily dispersible high transparency pigments.